Gooey / Transparent Material
Slime:

Inner object:


You'll have to adjust the settings to suit your model and lighting. I used a couple of lamps, only one that cast shadows. Increasing the 'Fac' of the mix shader on the inner object material makes it brighter, and the two in the slime material control the slime's brightness and transparency. (Left is transparency, right is brightness.)

I used lamps with a size of zero to make the shadows sharper.